Modern wireless infrastructure communication networks (e.g. 3G and 4G mobile communications networks) heavily employ multiple-input and multiple-output (MIMO) transmit (TX) and receive (RX) signal paths to maximize the capacity of each base transceiver station (BTS). The number of signal paths in a given direction (i.e. RX or TX) is typically large enough that they cannot feasibly be co-integrated in a single radio frequency integrated circuit (RFIC). In addition, integration of all RX signal paths or TX signal paths on a single RFIC is ordinarily not practical or commercially viable due to the limitations in state of the art integration capabilities for RFIC designs, and furthermore as it remains desirable to support flexible MIMO array sizes.
A known MIMO transceiver 10 is shown in FIG. 1. By way of example, the transceiver has four TX signal paths 12 labeled “TX Path X”, and likewise four RX signal paths 14 labeled “RX Path X”, where X=A, B, C, or D. It will be appreciated, however, that any suitable number of TX signal paths or RX signal paths may be provided, and that four are shown in FIG. 1 by way of example only. Each TX signal path 12 and each RX signal path 14 is generally embodied in a corresponding circuit which may be a printed circuit board (PCB) or an RFIC. Ordinarily, each TX signal path 12 outputs to a corresponding power amplifier 16 which is connected to a corresponding coupler 18 and thence to a corresponding antenna 20. Also connected to each coupler 18 there is ordinarily provided a corresponding low noise amplifier 22 leading to a corresponding one of the RX signal paths 14. Digital baseband TX and RX signals are communicated to and from a digital front end (DFE) 28 coupled to the TX signals paths 12 and the RX signals paths 14.
The MIMO signal processing techniques employed in such transceivers often rely on a fixed phase alignment amongst all RX signal paths and amongst all TX signal paths. Where each RX signal path is integrated on a corresponding RX RFIC, for example, the RX phase alignment must therefore be achieved across multiple RX RFICs, and similarly for the TX signal paths.
Aligning the phase of each unique RX signal path to the others in the RX array, and each unique TX signal path to the others in the TX array, is a significant challenge. In some applications, it is desirable to achieve phase alignment within accuracies of under 1° of the RF carrier. At radio frequency (RF) carrier frequencies sometimes employed in such communication networks (e.g. typically 1-3 GHz), 1° of RF phase corresponds to the order of 1 ps in units of time. Achieving this degree of alignment between multiple independent RX signal paths on the one hand, and between multiple TX signal paths on the other hand, is usually only practically achievable by employing board level calibration and alignment activities to calibrate all manufacturing variability and mismatches between each unique RX and TX signal path that makes up the overall MIMO transceiver array.
In order to achieve the desirable phase alignment between multiple separate RX signal paths that employ separate RX signal path circuits, a common strategy is to generate locally an alignment signal which sweeps across the RX frequency band in order to calibrate any differences in the phase delay of separate RX signal paths that are variable across the frequency band of interest. This alignment signal may be monitored by all of the local RX signal paths, and by comparing the received signals the phase alignment for each RX signal path can be measured and compensated for by the digital front end.
For this purpose, the MIMO transceiver 23 shown in FIG. 2 is similar to the MIMO transceiver 10 shown in FIG. 1 except that it is provided with a dedicated calibration transmitter 24 coupled to a corresponding power amplifier 26 and antenna 27. The calibration transmitter 24 generates an RX phase alignment signal which, via power amplifier 26 and antenna 27, is transmitted and thence received by antennas 20 and eventually RX signal paths 14, and is used by the digital front end 28 to calibrate the phase alignment amongst the RX signal paths 14.
While this approach allows RX phase alignment signals to be generated without disturbing the normal functional mode TX signal path alignments, it involves additional component cost and additional size of the transceiver. Although this strategy may have minimal impact on very large arrays (e.g. 8×8 MIMO arrays or larger) inasmuch as only a single alignment signal generator would be required and shared across the entire array of RX signal paths, the impact remains non-negligible. Of course, the impact is more significant on smaller MIMO arrays (e.g. a 4×4 MIMO transceiver).
There remains, therefore, a need for a technique for generating an RX phase alignment signal for phase alignment of TX and RX signal paths in MIMO transceivers which overcomes the above-described disadvantages of known approaches.